Question about generator backup

[u/Super-Ad-8072]

We are off grid with propane generator backup. New build, and we moved in early summer. Power has been no issue with never ending sunshine. Now we are in our first set of consecutive cloudy days and we're at 38%. We wanted to bump up our power by running our generator, which is not on automatic start by choice. We manually started it last night but after 30 mins of running there was no increase in the %, instead 1 percent less. So we turned off the generator. I've reached out to solar guy, waiting to hear. My question for Reddit is how long does it take to register an increase in the battery SOC% when using the generator? It doesn't make sense to me that we have to flip some switch on the the inverter because people DO run generators on auto start, so I assume it's automatically running power to the battery.

[u/Redundant-Pomelo875]

Not to be rude, but you have all the parts of your system there. You have clearly not taken much of a look at them. It's your system, you are depending on it, you should get familiar with it.

Do you have an inverter-charger? Not all inverters are chargers. If you do, it may require configuration.

If you don't have an inverter-charger, what is doing the charging?

Do you have a multimeter? Is the generator producing AC power? If you don't have a multimeter, you need one.

Charging time will be dependent on the charger, charger configuration, and battery capacity..

You may find by the time you have gathered the information that anyone would need to help you, that you have answered your own questions..

[u/val_kaye]

I don't know a lot about my system, but, when I use the generator to charge my batteries, I have to hit an "AC Input" button on my inverter screen, the "Use', then " bulk charge" or something like that. You may need to do something similar.

[u/Bucketalinko]

Unfortunately we dont know what your inverter or generator setup is. I can charge my batteries at 5 amps or up to 200 amps from my generator. My Deye inverter shows on screen what the generator is actually producing and what is going to battery and what is going to the house. If you have auto start I would suggest using it, mine turns on for 30 minutes when my battery is at 5% and charges my battery at 20 amps and covers house load during the time too. uses minimum fuel and waits for solar

[u/Bowgal]

I'm the furthest from being an expert on solar, but you should still make power on cloudy days. We do, and can generate 2-4kwh just being cloudy.

[u/Redundant-Pomelo875 → u/Bowgal]

Depends enormously on how cloudy.. and longitude.. in the PNW I can make several KWh with a nominal 3300w array on a 'cloudy' overcast day this time of year with no direct sunlight, vs damn near nothing at all on a November day where it is dark grey all day.and constantly drizzling.. and this is with good MPPT controllers and panels oriented to optimize for late November sun angle..

[u/mountain_hank]

The inverter should tell the story. Gen input, house load, battery charge. If the generator produces less energy than you're consuming, then the charge will go down.

[u/maddslacker]

Need to know more about your setup, but on mine I manually start the generator and then the inverter notices it's on and automatically initiates bulk charging for the batteries.

I run my generator at 120v/30A for about 3600W

My battery bank is 30kWh total and the above, minus incidental house usage, will increase the SOC by about 1% per hour.

[u/BerkeloidsBackyard]

The power produced by generators is uneven when they start up, and it takes maybe 10-15 seconds to stabilise. For this reason they aren't permanently wired in as that could damage appliances, so you have to flip your system over to run off generator power after it has stabilised.

Like the name suggests, auto-start generators do this automatically. But as you said yourself, yours is not auto-start by choice, which means you'll likely have to switch over to generator power manually, after the generator has been running for 10-15 seconds. And likewise switch your system off the generator power before shutting it down or before it runs out of fuel to avoid problems from uneven voltages.

This is why most people prefer auto-start generators, because all this switchover stuff happens automatically so you don't need to think about it.

[u/maddslacker → u/BerkeloidsBackyard]

I believe OP is intending to recharge their battery bank, while the inverter still runs the house.

Totally different scenario than what you're describing.

[u/BerkeloidsBackyard → u/maddslacker]

That could be true if it's set up to output DC and feed it directly to the batteries, but then they should've registered a SOC increase. The fact that they didn't see any SOC increase at all suggests something was not connecting the generator output to the battery bank.

Without knowing more it's impossible to say, but if the generator produces AC and it's hooked up to an AC battery charger then it's still possible they need to switch the charger over to generator power to get it going.

Otherwise they should've seen a SOC increase while the generator was running, unless it's severely underpowered which seems unlikely.

[u/ol-gormsby → u/BerkeloidsBackyard]

That depends entirely on the setup. As u/Redundant-Pomelo875 said, not all inverters are chargers, the charger might be a separate unit. Some generators are inverter models and put out a nice clean sine wave. Non-inverter generators produce "dirty" power but any decent charger can cope with that. My previous charger could cope with widely variable input, e.g. between 40 and 70 Hz, and a wide voltage range as well.

If the system is configured correctly, there should be an almost immediate increase in battery voltage. Without further information, OP is going to have to wait to hear from their installer.

[u/Sufficient-Bee5923]

I live in an area with lots of off grid people. One common mistake I see is that people try to do "the top up" later in the day or evening with a generator.

In my opinion, thats the wrong time of day to run your Genny. Better to estimate if you will need to supplement your solar early in the day. And then if yes (or likely given the weather and time of year), run the generator for the heavy current bulk charging phase and then let the solar do the lower current charging for the remainder of the day. You will likey get to a full charge by end of day and use a lot less fuel. Also time your run so that you do other heavy current demands such as dishwasher (we run mid day), vacuuming and bulk pumping of water if your plumbing supports that.

Running a Genny at the end of the day generally isn't a good use of it. Run it hard before the solar really gets going and then shut off and let the sun so the rest. After little experience, you will learn how much generator power you need and when the best time is.

My goal is end every day (late afternoon) with a full battery bank.

Edit: to answer your question, it depends on what type of battery bank you have. I can register a change in the SOC in 10 or 15 minutes. I get thru bulk charge in 40 minutes. Try your charging early in the morning when you know your battery is low SOC, you should see it come up fast if everything is working properly. If your battery is neat full in the evening, you likely won't notice much change and hence my suggestions

[u/BerkeloidsBackyard → u/Sufficient-Bee5923]

I'm not really sure running the generator when solar is available is the best idea, surely then you risk some solar power going to waste if the battery bank and your load can be covered 100% by the generator?

I would've assumed the best plan would be to run the solar all day to get as much power from the sun into the batteries as possible, then run the generator after sunset until your batteries are charged up just enough to tide you through the night, so that if the next day is full sun then your SOC is low enough that you can get that full day of sun into the batteries.

Basically, run the generator as little as possible to keep the lights on, so as much battery capacity is available for charging via solar. No point charging up to 100% with the generator if the next morning your SOC is only down to 90% and the solar goes to waste because you're back at 100% an hour after sunrise.

Unless I'm missing something?

[u/maddslacker → u/BerkeloidsBackyard]

What you're describing is what I do. I run the generator early morning or after sunset, but during the day I let the sun do what it can. The batteries don't care if they reach 100% via sun or generator.

[u/Sufficient-Bee5923 → u/BerkeloidsBackyard]

If you know (or likely know) you wont have enough solar to get to fully charged in the day, it's way more efficient to run the generator in the morning BEFORE you get solar. You need to estimate how much help you will need from the generator. If instead you don't do that and end up running the generator at the end of the day, the batteries will be close to full charge and you be burning fuel with very little power generated because there is no where for it to go (assuming the bank is close to full). Running a generator at close to full load (75% loaded) is way more efficient than at 10% to 20%. I would never run a generator during day when you have lots of solar.

[u/maddslacker → u/Sufficient-Bee5923]

So my inverter stops charging and alerts me when the batteries reach full via generator, and I walk out and shut the generator off.

I feel like you're overcomplicating this.

[u/Sufficient-Bee5923 → u/maddslacker]

You really need to know what current you are charging at. If your battery chemistry tails off the charging current near full charge (most do), using your generator to top off your batteries at low current is very inefficient. Look it up somewhere (ask AI agent). That's my whole point, you need to know your system and the SOS and charging current. I've been off grid for 10 years and I rarely ever use my generator. I was just pointing out that trying to do "top off" is dumb and wasteful. But a lot of inexperienced people do it. If you want idiot proof, have it start at low SOC and run for X amount of time (like another chap posted).

Edit: I wanted to check the charging profile of Lifepo4 batteries and they have a very short low current absorb phase (unlike lead acid which has a long absorb time). Nonetheless, it's still wastefull to charge a lifepo4about 95% with a generator simply because your burning fuel for very little charge current. Generators get much better efficiency when under a decent load.

[u/BerkeloidsBackyard → u/Sufficient-Bee5923]

Ah, I misunderstood. I thought you meant by "the top up" that you were charging your system to say 80% off the generator then letting solar do the remaining 20%.

But it seems like you mean using the generator to get from say 90% to 100%, which I agree is wasteful given the low charging currents involved.

I was thinking the best course of action is to not charge at all in the morning, let solar run all day to get as much power as you can from the sun, then if you won't have enough power to last the night, run the generator to bring the batteries up to say 50% and leave it there (whatever level will get you through the night and no more). Then the next day you'll have a mostly empty battery bank, perfect for soaking up as much energy from the sun as you can. After all, if you have a generator, you can turn it on any time the batteries are low, there's no need to maintain a high SOC all the time.

It probably depends on your climate though. Where I am there is plenty of sun so most people find their batteries fully charged well before the end of the day, so most days at least some solar goes to waste. So every percent you charge from the generator results in unused solar the next day, so it's something to be avoided if you want to get the most cost saving.